Budraitis and Seliga U.S. Pat. Nos. 4,195,563 and 4,274,282 show a load monitor with a load limit comparator and a load indicating meter which may be either an analog meter or a digital meter. Upon occurrence of an overload the machine is stopped and the meter indicates the load to which the machine was subjected. The peak load detector is an analog circuit utilizing a capacitor which is charged to the peak signal value. The charge on the capacitor decays over a period of time. With an analog meter, the reading becomes progressively inaccurate. A digital meter may be used and latched on occurrence of an overload so that the peak overload is indicated indefinitely. However, many operators find analog meters easier to read accurately than digital meters.
The load limit circuits of the Budraitis and Seliga patents have comparators which compare the load experienced during each cycle of the machine with reference signals. Typically, the reference is set by an operator adjusted potentiometer.
The load on a machine depends on the nature of the tooling and material. In setting up a machine for manufacture of a part, experience and care are required to select the load limits and to adjust the limit potentiometers properly. If the overload reference is set too low, the machine will shut down unnecessarily. If it is set too high, the tooling and machine may be damaged. Conversely, if the low load comparator is set too low, the machine may continue to run without stock or with a broken tool and if it is set too high, operation may be interrupted unnecessarily.
Dybel et al. U.S. Pat. No. 4,171,646 shows a monitor which establishes a load limit comparator reference after several cycles of machine operation and which changes the reference as the machine operates. A machine utilizing this control must be watched carefully as if the tooling dulls or the stock changes character slowly, the load limit will change and the machine continue to operate until the absolute overload circuit is actuated. The tooling or machine may be damaged or unusable product may be produced.